The present invention generally relates to automatic equalizers, and more particularly to an automatic equalizer which compensates for a line distortion adaptively to the transmission characteristic of the line in a communication system which makes a digital transmission.
In a communication system which transmits digital transmission information via an analog line, a modem is provided at both the transmitting end and the receiving end for consistency of the transmission information and the line. The modem at the transmitting end sends a training signal prior to starting transmission of the transmission information. On the other hand, the modem at the receiving end carries out a series of initial pull-in control including reproduction of the carrier and receiving timing based on the received training signal to establish synchronization, and adjustment of an automatic equalizer adaptively to the transmission characteristic.
FIG. 1 shows an example of a conventional automatic equalizer. In FIG. 1, a received signal is supplied via a transversal filter 31 to a transmission information processor (not shown) and to one input of a multiplier 32 which are provided at the subsequent stage. An output of the multiplier 32 is supplied to an input of a discriminator 33 and to one input of a subtractor 34. An output of the subtractor 34 is supplied to an input of a carrier adaptive phase controller (CAPC) 35 and to one input of a multiplier 36. One output of the CAPC 35 is supplied to the other input of the multiplier 32, and the other input of the CAPC 35 is supplied to the other input of the multiplier 36. An output of the multiplier 36 is fed back to a feedback input of the transversal filter 31.
The transversal filter 35 includes delay elements 37.sub.1 through 37.sub.N which are connected in series, multipliers 38.sub.1 through 38.sub.N+1, and an adder 39. The received signal is supplied to the delay element 37.sub.1. A plurality of taps provided at the input and output ends of the delay elements 37.sub.1 through 37.sub.N are respectively coupled to inputs of the adder 39 via the multipliers 38.sub.1 through 38.sub.N+1. An output of the adder 39 is supplied to the transmission information processor and to the input of the multiplier 32, as described above.
In FIG. 1, it is assumed for the sake of convenience that a received signal X.sub.n is supplied to the transversal filter 31, where n increases with time. In addition, it is assumed that an output signal Y.sub.n is output from the transversal filter 31 in response to the received signal X.sub.n. In this case, the multiplier 32 outputs a rotation signal which is obtained by shifting the phase of the output signal Y.sub.n by .theta. radians depending on the phase error information exp(j.theta.) of the carrier which is detected by the CAPC 35. The discriminator 33 outputs a reference signal Z.sub.n which indicates a reference point which is closest to the above rotation signal out of all the reference points in the signal space which is dependent on the modulation technique of the received signal X.sub.n.
The subtractor 34 outputs an equalizer error signal which indicates an error of the rotation signal with reference to the reference signal Z.sub.n. The multiplier 36 shifts the phase of the equalizer error signal by -.theta. radians depending on the inverted phase error information exp(-j.theta.) which is output from the CAPC 35. In other words, the multiplier 36 outputs a reference error signal E.sub.n by cancelling a phase shift component which is given to the signal Y.sub.n via the multiplier 32.
In the transversal filter 31, values of coefficients C.sub.(-N/2) through C.sub.(N/2) are varied depending on the reference error signal E.sub.n. Hence, the amplitude distortion and the phase distortion of the received signal X.sub.n are compensated simultaneously depending on the transmission characteristic of the line.
In a mode which uses the above described automatic equalizer, the equalizer characteristic may greatly shift with respect to the line characteristic and diverge for some reason during reception of the transmission information after the initial pull-in control is completed. In such a case, the modem, in general, requests the modem at the remote end to retransmit the training signal in conformance with a predetermined communication procedure. Thereafter, the modem carries out the initial pull-in control again based on the training signal which is received from the remote modem in response to the retransmission request.
However, the modem may be applied to a multi-point communication system shown in FIG. 2 which includes a master station 41 and a plurality of slave stations 42.sub.1 through 42.sub.M which are connected in parallel to a single 2-wire line which connects to the master station 41. In other words, the automatic equalizer may be applied to the modem of each slave station. But in such a case, the automatic equalizer must retry the initial pull-in control every time the automatic equalizer diverges. In order to retry the initial pull-in control, the training signal and the like must be exchanged via the line which is used in common by the plurality of slave stations 42.sub.1 through 42.sub.M. As a result, the transmission efficiency of the entire multi-point communication system deteriorates.
Accordingly, in order to avoid the transmission efficiency from deteriorating due to the retrying of the initial pull-in control, the conventional automatic equalizer carries out the initial pull-in control based on a received signal which is modulated by the transmission information, instead of using the training signal.
If the number of reference points in the signal space corresponding to the modulation technique of the received signal, the conventional automatic equalizer can obtain the reference signal Z.sub.n from the received signal which is modulated by the transmission information with a high reliability, because the distance between two reference points is long.
However, if a high-speed digital modulation technique with the transmission rate of 14.4 kbs is employed, the number of reference points becomes 128 and large. In this case, the distance between two reference points becomes short and the reliability of the reference signal Z.sub.n becomes poor. Consequently, the reliability of the reference error signal E.sub.n, which is used as a reference for setting the coefficients in the transversal filter 31, also deteriorates. For this reason, there is a problem in that the initial pull-in control becomes unstable and difficult.